Generally speaking, to read the electric current coming from a sensor, a two-phase method needs to be adopted, namely:                an integration phase, during which the moving charges coming from the sensor are directed towards a capacitive element so that they can be accumulated therein,        and a read phase as such, during which the capacitive element is discharged so that the previously accumulated moving charges can be directed to an output stage, in which said current, resulting from the moving charges, is processed.        
In the particular case of a photosensitive sensor, the moving charges are liberated during the interactions between photons and matter which occur in a photosensitive element making up the elementary sensor or sensors that constitute the electromagnetic radiation detector. Electrical devices for reading such currents therefore act as the integration interface between the sensors of the detector and the output stages, which constitute the circuit for exploiting the integrated signals.
From the prior art, different architectures are known for designing a read device of this kind. Read device is taken to mean, in the context of the present invention, the input stage of the circuit for reading and exploiting the signals delivered by the detector.
Indeed, in a known way, a read and exploitation circuit comprises an input stage directly connected to the detector, and an output stage receiving the signals integrated by said input stage.
Among prior art read devices, the device is known which is shown in FIG. 1 and generally known as a direct injection input stage. A read device or input stage of this kind comprises a polarization transistor 101 and an integrating capacitive element 102.
As shown in FIG. 1, the source of the transistor 101 is connected to the input branch intended to inject into this input stage a current Ie for reading, coming for example from a photodiode, subjected to an incident radiation flux. The drain of the transistor 101 is itself connected to a terminal of the capacitive element 102, and to a terminal connected to the output stage. These three terminals are therefore brought to the output potential Vs. The other terminal of the capacitive element 102 is connected to a reference potential V_, generally the potential of the mass. Moreover, the gate of the transistor 101 is connected to a polarization voltage source, which determines the level of polarization of the detector across the transistor 101.
In operation, the moving charges liberated during the interactions between photons and matter occurring in the sensor located upstream of the input branch generate the current for integration Ie and they accumulate on the capacitive element 102. This charge build-up therefore equates to the so-called “integration” phase.
At the end of the integration phase, the integrating capacitive element 102 may be read and discharged. The resulting signal is then directed to the output stage and the exploitation circuit. This is the current “read” phase itself.
Architecture of this kind is commonly known as a direct injection input stage. Depending on the way it is implemented, said architecture may be broken down into different versions.
FIG. 2 shows for example the addition of a selection transistor 203 on the output branch of the input stage shown in FIG. 1. The selection transistor 203 allows a plurality of input stages to be put in parallel for a single output stage. The transistor 203 thus performs a selection function and can be used to multiplex a plurality of parallel input stages.
As a rule, designers seek to miniaturize photo-detector components as much as possible in order to increase the resolution, and therefore the number of pixels formed on the image surface. This constraint on bulk concerns not only the elementary sensors themselves, but also the electronic circuits that provide the interface between the detector and its exploitation circuit, namely the input stages and the output stages.
Apart from this constraint on bulk which is crucial, photo-detectors must generally consume as little power as possible and have very short reaction and integration times. The power consumption problem proves particularly significant in the case of cooled photo-detectors, such as diode detectors for the infrared radiation spectrum for example.
It is for this reason that integrated circuits constituting current read devices are generally deemed to have of necessity to comprise few electronic components and offer a relatively straightforward architecture. This then means that the space they require and the power they consume, and therefore the Joule heating thereof, can be reduced.
Prior art integrated circuits, such as those shown in FIGS. 1 and 2, respect these bulk and overheating constraints, but need to be completed to provide reset and integration blocking functionalities.
To provide reset, the proposal has been made to produce a direct injection input stage, offering the integrated circuit architecture shown in FIG. 3. To the core architecture of the circuit in FIG. 1 is added a transistor 305 the function of which is to reset the integrating capacitance 302 prior to each integration phase. Furthermore, in the context of the input stage shown in FIG. 2, the integrating capacitance 202 may be reset when the input stage is selected using the selection transistor 203.
The input stages shown in FIGS. 1 and 2 have furthermore the drawback of being sensitive to bloom, which even so causes the sensor to depolarize or a parasitic current to appear in the multiplexing system. Blooming is produced when the “hot” source detected by the sensor is too intense, and saturates said sensor.
It is necessary as a result to add an anti-blooming transistor 304 to integrated circuits of this kind, thereby increasing the number of electronic components, and therefore the bulk of the read device.
To block integration, in the case of the input stages shown in FIGS. 1, 2 and 3, a known solution is the sampling of the wanted signal in the pixel. To do this, a transistor 406 and a storing capacitive element 407 are added in a known way in order to sample the voltage at the terminals of the integrating capacitance 402. The capacitive element 407 stores the signal in the pixel and authorizes the start of the next integration during the sequential read. It is thus possible to integrate the charges during the sequential read, which gives a time gain for the integration phase.
However, adding components of this kind, and in particular the capacitive element 407, necessarily increases the bulk of the read device, as well as its power consumption.
To partially overcome these drawbacks, a capacitive element 407 of small dimensions, and therefore of small capacity, is generally implanted, as well as a transistor 406 of size reduced to the minimum.
However, sizing the components 406 and 407 in this way causes a degradation of the wanted signal, in other words of the integrated current, since there is dilution of the wanted signal, increase in noise and injection of non-linear charges in the transistor 406.
The value of the dilution of the wanted signal is
      1    -                  C        402                              C          402                +                  C          407                      ,where C402 and C407 are the values of the capacitances of the capacitive elements 402 and 407 respectively.
The quadratic noise contribution is furthermore equal to
      kT          C      407        ,where T is the temperature and k the Boltzmann constant.
Such a degradation of the wanted signal inevitably involves a decrease in the signal-to-noise ratio, and therefore in the performance of the photo-detector provided with an input stage of this kind.
The injection of non-linear charges itself degrades the linearity of the wanted signal when the wanted charge stored in the capacitive element 407 is small.
Another known solution for blocking integration comprises blocking the polarization transistor in order to define a windowing.
FIG. 5 thus shows a direct injection input stage, with the windowing function thereof being provided by means of a polarization transistor 501, the control or gate voltage of which is of the impulse type. In this way, depending on the form of the current pulse of the polarization voltage, the current for reading coming from the detector is switched to the integrating capacitive element 502. However, switching the current for reading in this way has two drawbacks.
First of all, there is instability in sensor polarization, under the effect of the polarization voltage pulses.
Indeed, the input stage must maintain a potential that is as stable or constant as possible at the sensor terminals, in order to effectively inject the currents for reading into the read device constituting the input stage.
But the input stage as shown in FIG. 5 does not allow a stable potential to be maintained at the terminals of the elementary sensor, when the polarization transistor is in the off state, in other words outside the integration phase.
Consequently, prior art input stages that use polarization transistor blocking cannot prevent the integration of parasitic charges into the capacitance, parasitic charges inevitably carried by the diode constituting the elementary sensor. An elementary sensor generally behaves like a diode that has shunt capacitance, like any semi-conductor.
Parasitic charges can be of miscellaneous origin, among which are dark current, or more generally parasitic photons as received by elementary sensors, etc. Quite obviously, such parasitic charges cause a drift in potential difference at the terminals of the elementary sensor and, consequently, degrade the linearity which constitutes an essential performance element in rating any photo-detector.
In addition, in the case of the circuit in FIG. 5, the transistor 501 doing the switching, generally of MOS technology, injects parasitic charges in variable quantity towards the integrating capacitive element 502. Consequently, as is the case in respect of sampling, this architecture has the drawback of degrading the linearity of the wanted signal.
These two drawbacks are particularly disadvantageous if the circuit is to be used defining a plurality of integration windows between two resets of the integrating capacitance.
The aim of the present invention is therefore to propose a current reading device that makes it possible to avoid the drawbacks afforded by the integrated circuits of prior art input stages, among which may be cited variations in sensor polarization, if not the depolarization thereof, the injection of parasitic charges into the integration circuit, a significant bulk, relatively high power consumption, and therefore relatively high Joule heating, the loss of time needed to reset the integrating capacitance, the risk of blooming or, conversely, the additional cost of implanting additional electronic components to deal with these different problems.